Aluminum-lithium alloys have distinct advantages over conventional aluminum alloys for use in weight sensitive applications such as aircraft manufacture. The combination of light weight with high structural integrity for Al-Li alloys in aircraft applications permits increased fuel savings. As a result of these advantages, major aluminum fabricators have worked to develop processes for manufacturing these alloys into useful wrought forms. The manufacturing of Al-Li alloys presents process control and maintenance problems due to the corrosive nature of the aluminum-lithium alloy and the tendency for lithium to volatilize. When aluminum and lithium are combined together, the corrosive nature is greater than either of the two components individually. As a result of these problems, aluminum fabricators add the lithium alloy addition to the aluminum melts as late as possible during the alloy manufacturing process to minimize any adverse effects.
Due to the increased use of aluminum-lithium alloys in aircraft manufacture, an increased amount of Al-Li scrap has been generated which must be recycled. However, reclamation and recovery of the aluminum-lithium scrap can present problems due to the corrosive nature of the material. Standard melting techniques under conventional salt flux layers can result in damage to the refractory linings of the furnace, as well as low yields due to loss of the valuable lithium component.
In response to these problems, processes have been developed to selectively remove the lithium from the aluminum-lithium scrap material so that the aluminum may be utilized for other purposes and the valuable pure lithium, costing on the order of $25.00/lb, may be reused as an alloy additive.
U.S. Pat. Nos. 4,780,186 and 4,973,390 to Christini et al. and U.S. Pat. No. 4,849,072 to Bowman disclose a method for recovering lithium from aluminum-lithium alloy scrap using an electrolytic cell. In these patents, an electrolytic cell is provided which utilizes the varying density of three phases: an Al-Li scrap alloy (heaviest), a molten salt (middle range), and lithium (lightest). An electric current is passed through the cell thereby reducing lithium ions to lithium metal so that lithium is removed from the uppermost layer in the three layer electrolytic cell. One drawback to this process is the operating temperature of the electrolytic cell. As opposed to standard lithium electrowinning cells, the cell temperature for these types of processes is 200.degree. C. higher. This higher temperature results in an electrorefining cell which will greatly promote oxidation and loss of any lithium which has been recovered. It will also enhance the lithium solubility in the molten salt causing an increase in the back reaction and current efficiency loss which will result in an overall extremely low current efficiency.
U.S. Pat. No. 4,790,917 to Dewing discloses another method of refining lithium-containing aluminum scrap using an electrolytic cell. In this method, an Al-Li molten alloy is used as an anode, a mixture of lithium chloride and other chloride salts is used as an electrolyte and either molten aluminum or lithium may be used as the cathode. The anode pool is separated from the cathode pool by a barrier and both pools are covered by the electrolyte. In one disclosed embodiment, the lithium from the scrap is transported to the pure aluminum pool and thus recovered as an aluminum-lithium alloy. The drawback for this process is that the recovered lithium is still in the form of a binary Al-Li alloy. Although the lithium may have been separated from other trace elements in the scrap, the recovered product still is in a highly reactive form. This product form limits the use of the recovered Al-Li product to recycle material for aluminum-lithium alloys or as a product requiring only the chemical reducing power of aluminum, such as a steel oxidizer. In addition, although the cell arrangement may have a low Nernst potential, the cell may have a high total cell drop due to the presence of the barrier and the bath IR drop may also be high. Another major drawback to this system is that since an Al-Li product is produced, a protective salt layer will be required to prevent oxidation when removing the product from the cell. Also, the process requires a source of pure aluminum when aluminum is used as a cathode.
None of these patents teaches or fairly suggests a method of extracting lithium from an aluminum-lithium alloy which utilizes an electrolytic cell and a pure molten tin pool as the cathode, thereby resulting in recovering the lithium from the Al-Li alloy in the form of a tin-lithium alloy. Furthermore, none of these patents teaches or suggests further recovery of the lithium from the produced Sn-Li alloy by methods such as electrolytic cells, vacuum distillation or chemical conversion.
U.S. Pat. No. 4,455,202 to Sintim-Damoa et al. discloses a method of producing lithium by electroreduction in a fused salt electrolyte. In this method, a liquid tin metal cathode is used to recover lithium from a lithium compound such as lithium oxide. The teachings of this patent are different from that of the present invention in that the Sintim-Damoa et al. reference does not disclose recovering lithium from an aluminum-lithium alloy.
U.S. Pat. No. 4,882,017 to Weaver discloses a method of making a light metal-alkali metal master alloy using an electrolytic cell. The teachings of Weaver are different from the present invention in that the inventive method is designed to recover lithium from scrap material rather than to produce a lithium alloy as is disclosed by Weaver.
U.S. Pat. No. 3,728,234 to Sakai et al. discloses an electrolytic cell that identifies tin as one of a number of liquid metals that may be used as a cathode. Again, Sakai et al. does not teach or suggest extracting lithium from aluminum-lithium alloys using a pool of molten tin as a cathode in an electrolytic cell.
In response to the deficiencies in prior art processes, a need has developed to provide methods for extracting lithium from aluminum-lithium alloys which extract lithium in a more usable and less volatile form. In response to this need, Applicant has developed a process which will be described hereinafter which extracts lithium from Al-Li alloys and produces a tin-lithium product which is much less reactive and more recoverable than prior art process products.